A brushless motor is used, e.g. as a driving motor of a direct-drive washing machine, and such a motor desirably spins at a low speed with large torque, and produces low vibration and low noise. The motor used as a direct driving motor has no gear and needs large torque, for it drives an object directly, so that the motor employs an outer-rotor structure and a stator wound with concentrated winding as shown in FIG. 1 and FIG. 4 which depict the reference patent document 1.
In general, a motor requiring a low rpm with large torque employs the concentrated winding. A motor with a distributed winding obtains more interlinkage magnetic flux in the winding than the motor with the concentrated winding, so that the motor with distributed winding tends to produce output torque greater than the motor with the concentrated winding by 10-15%.
However, the motor with the concentrated winding can have a coil end smaller than that of the motor with the distributed winding, so that it can reduce a resistance of the winding, and from a total view of point, the motor with the concentrated winding produces heat lower than the motor with the distributed winding when they need the same output torque. Since its coil end can be smaller than that of the other, the volume of the motor can be reduced. It can be thus concluded that the motor with the concentrated winding is good for an application that needs large torque at a low rpm, and requires in particular a downsized body. At a high rpm, this motor also employs field-weakening control (a phase-advancing driving method), so that an electric current phase is driven with an advancing phase angle.
However, the motor with the concentrated winding produces greater radial force than the motor with the distributed winding, so that it produces greater vibration and noise. FIG. 7A shows a simulation of a single-rotor motor with the concentrated winding, and FIG. 7B shows a simulation of a single-rotor motor with the distributed winding. These Figures tell that the motor with the concentrated winding produces substantially greater radial force than the motor with the distributed winding. Since the motor spins at a low speed, it tends to be subject to the influence of cogging torque. The motor spinning at a low rpm with large torque and used in a direct-drive washing machine is thus required to produce small cogging torque and small radial force.
Reference patent document 2 discloses a motor having two rotors, namely, an inner rotor and an outer rotor. Hereinafter this motor is referred to as a double-rotor motor with concentrated winding. This double-rotor motor with concentrated winding has the following structure
divided teeth wound with concentrated winding, and which teeth are coupled together by molding for forming a stator; and
two rotors, each of which inside and outside are stuck with permanent magnets, and having a uniform space between the stator. (Refer to FIG. 8 that depicts the reference patent document 2.)
The permanent magnets stuck to the inside have different poles from those stuck to the outside, so that the magnetic flux travels from the outer rotor to the inner rotor via the teeth of the stator, and returns to the outer rotor via the teeth of the stator, i.e. it forms a loop. Since this double-rotor motor with concentrated winding can use the magnetic fluxes traveling through both inside and outside rotors, it can produce a greater output density than a conventional single-rotor motor. However, the double-rotor motor still employs the concentrated winding as the single-rotor motor does, it produces greater radial force, so that its vibration and noise still remain great.
Reference patent document 3 discloses a motor having two rotors, namely, an inner rotor and an outer rotor, and its stator is wound with toroidal winding. FIG. 8 shows a sectional view of this motor. This motor comprises the following elements: stator 110; inner rotor 120; and outer rotor 130, to be more specific, the motor is a double-rotor with toroidal winding, and has 8 poles and 12 slots.
Stator 110 is formed of stator yoke 114, outer teeth 112 and inner teeth 113 both provided to stator yoke 114. Stator yoke 114 is wound with three-phase coils 115. In general, coils 115 are coupled together in a manner of star-shaped wire connection or a delta-shaped wire connection.
Inner rotor 120 is rotatably held inside stator 110 and is formed of inner rotor yoke 121 and inner permanent magnets 122. Outer rotor 130 is rotatably held outside stator 110 and is formed of outer rotor yoke 131 and outer permanent magnets 132. Inner rotor 120 and outer rotor 130 are driven with the magnetic field produced by the current running through coils 115. FIG. 8 shows a surface-magnet rotor, i.e. permanent magnets 122 and 132 are mounted on the surfaces of inner rotor 120 and outer rotor 130 respectively.
FIG. 9A shows an induction voltage waveform actually measured with respect to a rotor position in the case of 24 poles and 18 slots. FIG. 9B shows an induction voltage waveform actual measured with respect to a rotor position in the case of 8 poles and 12 slots. The X-axis of each case represents a rotor position in electric angles. FIGS. 9A and 9B tell that the induction voltage waveforms are distorted asymmetrically because of buffer action between outer rotor 130 and inner rotor 120, and such a distortion in the induction voltage substantially increases the vibration and noise.
The foregoing prior art proves that use of two rotors allows increasing the output torque; however, e.g. the ratio of the number of slots (S) vs. the number of poles (P), S:P=3:2N (N is an integer equal to 1 or more) will make the winding configuration equal to that of the concentrated winding, so that the radial force becomes greater and thus the noise tends to increase.
In the case of a regular single-rotor motor with distributed winding, the structure allows canceling out the radial force; however, the structure enlarges the coil end, so that the resistance of the winding increases, which lowers the efficiency and physically enlarges the motor.
Reference Patent Document 1: Examined Japanese Patent No. 3725510
Reference Patent Document 2: Japanese Translation of PCT Publication No. 2005-521378
Reference Patent Document 3: Unexamined Japanese Patent Publication No. 2001-37133